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The present invention is directed to a ferritic stainless steel alloy. More particularly, the present invention is directed to a ferritic stainless steel alloy having microstructural stability and,.mechanical properties making it particularly suited for high temperature applications. Such applications include, but are not limited to, current collecting interconnects in solid oxide fuel cells, furnace hardware, equipment for the chemical process, petrochemical, electrical power generation, and pollution control industries, and equipment for handling molten copper and other molten metals.
Fuel cells are highly efficient, environmentally friendly means for generating electric power. The basic principle behind the operation of fuel cells is the generation of electricity by the combustion of fuel. The fuel is separated from an oxidizer by a permeable barrier known as an electrolyte. Hydrogen atoms on the fuel side of the electrolyte are ionized. The resulting protons pass through the electrolyte, while the liberated electrons travel through an external circuit. On the air side of the electrolyte, opposite the fuel side, two protons combine with an oxygen atom and two electrons to create a water molecule, liberating heat and completing the electric circuit. Energy is extracted from the process by using the electrons in the external circuit to do work. For fuel cells which run at higher temperatures, heat liberated from the reaction on the air side can also be used for fuel reformation or heating applications, increasing the efficiency of the cell""s overall operation.
A type of fuel cell currently attracting much interest is the solid oxide fuel cell (SOFC). SOFC""s operate at high temperatures (1450-1800xc2x0 F. (788-982xc2x0 C.)), which means that they can internally reform common hydrocarbon fuels such as natural gas, diesel fuel, gasoline, alcohol, and coal gas into hydrogen and carbon monoxide. Internal reformation recycles thermal energy and eliminates the need for expensive platinum group metal catalysts. Hydrogen and carbon monoxide are both used as fuel in the SOFC. Hydrogen combines with oxygen in a modification of the generic fuel cell reaction detailed previously. The electrolyte is an oxide ceramic, which is permeable to oxygen ions (O2xe2x88x92), rather than to protons. Thus, the SOFC runs in a reverse direction relative to certain other fuel cell types. In addition to combusting hydrogen, carbon monoxide is oxidized to carbon dioxide at the anode, releasing heat. This is an advantage because, carbon monoxide is present in unrefined fuels and can poison low temperature fuel cells, reduce operating efficiency. Small SOFC""s operate at up to about 50% efficiency. To achieve even greater efficiency, medium sized and larger SOFC""s can be combined with gas turbines. The resulting efficiency of a combined SOFC-gas turbine set can reach 70%.
Several variants on the basic SOFC design exist. The electrolyte is typically a form of zirconia that has been stabilized by the addition of oxides to inhibit lattice changes and provide high ionic conductivity when heated to high temperatures. Such oxide-stabilized materials are generally known, and are referred to herein, as xe2x80x9cstabilized zirconiaxe2x80x9d. SOFC""s commonly include yttria-stabilized zirconia (YSZ) as the stabilized zirconia electrolyte. A reported coefficient of thermal expansion (CTE) of YSZ, between 20xc2x0 C. (68xc2x0 F.) and 1000xc2x0 C. (1832xc2x0 C.), is about 11xc3x9710xe2x88x926 per xc2x0 C.
A tubular SOFC, of relatively simple construction, which operates at extremely high temperatures (1800xc2x0 F. (982xc2x0 C.)) and is large in size, has been developed. A tubular SOFC may be scaled up in size by increasing the size and number of individual SOFC tubes in the device. More recently, the xe2x80x9cplanarxe2x80x9d SOFC (PSOFC) has been developed. PSOFC""s are relatively compact and are constructed of stacks of flat cells. The anode and cathode plates are typically ceramic materials. Permeable nickel-zirconia cermets have also been used for the anode.
Interconnects are needed to collect the electrons generated by a fuel cell. Interconnects also function as a physical separator for the oxidizing and, reducing gas streams. Accordingly, the material used to form fuel cell interconnects should be electrically conductive, oxidation resistant, and mechanically stable, and should have thermal expansion properties substantially matching those of the ceramic components of the cell, which may be physically disposed adjacent to the interconnects. Until recently, SOFC interconnects were commonly fabricated from ceramic material that is electrically conductive at high temperatures, commonly LaCrO3 doped with either CaO or SrO. Although ceramics typically are stable when subjected to high temperatures for prolonged periods, ceramics also are brittle and relatively expensive, and are poor conductors of electricity relative to metals. Certain metallic interconnects have been fabricated from a chromium-based alloy developed for that purpose. The alloy provides adequate oxidation resistance and a good thermal expansion match with stabilized zirconia. However, the powder metallurgical route used to produce the alloy makes it very expensive, which adds substantial cost to SOFC""s produced from the alloy.
Fabricating SOFC interconnects from stainless steels may provide advantages over ceramics because the steels would have greater electrical conductivity and may be in a form less brittle than ceramics. However, problems associated with the use of stainless steels in SOFC interconnect applications include oxidation, thermal expansion, and creep problems. Oxidation can reduce the capacity of a stainless steel to conduct current, thereby reducing cell output over time. Standard austenitic stainless steels do not provide a good thermal expansion match with conventional SOFC electrolyte ceramics. Ferritic stainless steels that may provide a good thermal expansion match with the ceramic electrolytes typically exhibit low creep resistance. For example, tests conducted by the present inventor on several commercially available stainless steels, including E-BRITE(copyright) (UNS S44627), AL 29-4-2(copyright) (UNS S44800) and ALFA-IV(copyright) (Alloy Digest SS-677, ASM International) alloys, have demonstrated that E-BRITE(copyright) alloy has acceptable thermal expansion for SOFC use, good thermal stability, and forms the desirable Cr2O3 oxide. The creep resistance of E-BRITE(copyright) alloy, however, is less than desirable for SOFC applications.
Thus, there exists a need for an improved stainless steel alloy having high temperature creep resistance, good.thermal stability, and other characteristics that make it suitable for use as current collecting interconnects in SOFC""s and for use in other high temperature applications, such as in equipment for the chemical process, petrochemical, electrical power generation, and pollution control industries, as well as in furnace hardware and equipment for handling molten metals.
The present invention addresses the above-described need by providing a ferritic stainless steel including greater than 25 weight percent chromium, 0.75 up to 1.5 weight percent molybdenum, up to 0.05 weight percent carbon, and at least one of niobium, titanium, and tantalum, wherein the sum of the weight percentages of niobium, titanium, and tantalum satisfies the equation 0.4xe2x89xa6(%Nb+%Ti+xc2xd(%Ta))xe2x89xa61. The steel of the present invention has a CTE within about 25% of the CTE of stabilized zirconia between 20xc2x0 C. (68xc2x0 F.) and 1000xc2x0 C. (1832xc2x0 F.). The steel of the present invention also exhibits vat least one creep property selected from creep rupture strength of at least 1000 psi at 900xc2x0 C. (1652xc2x0 F.), time to 1% creep strain of at least 100 hours at 900xc2x0 C. (1652xc2x0 F.) under load of 1000 psi, and time to 2% creep strain of at least 200 hours at 900xc2x0 C. (1652xc2x0 F.) under load of 1000 psi.
The present invention also provides a method for making a ferritic stainless steel alloy wherein the method includes forming a steel comprising greater than 25 weight percent chromium, 0.75 up to 1.5 weight percent molybdenum, up to 0.05 weight percent carbon, and at least one of niobium, titanium, and tantalum, wherein the sum of the weight percentages of niobium, titanium, and tantalum satisfies the equation 0.4xe2x89xa6(%Nb+%Ti+xc2xd(%Ta),)xe2x89xa61. The steel has a CTE within about 25% of the CTE of stabilized zirconia, and preferably has a CTE that is greater than and within 25% of the CTE of stabilized zirconia, between 20xc2x0 C. (686xc2x0 F.) and 1000xc2x0 C. (1832xc2x0 F). The steel also has at least one creep property selected from creep rupture strength of at least 1000 psi at 900xc2x0 C. (1652xc2x0 F.), time to 1% creep strain of at least 100 hours at 900xc2x0 C. (1652xc2x0 F.) underload of 1000 psi, and time to 2% creep strain of at least 200 hours at 900xc2x0 C. (1652xc2x0 F.) under load of 1000 psi. In a subsequent step, the steel is solution annealed and then cooled from the annealing temperature. Solution annealing preferably is performed at a temperature that is at least the greater of the intended service temperature of the alloy and 1600xc2x0 F. (871xc2x0 C.). If desired, the solution annealed stainless steel is precipitation heat treated to harden the steel.
The present invention also provides for the fabrication of articles of manufacture from the stainless steel of the present invention. The articles may be fabricated using conventional techniques.
The stainless steel of the present invention exhibits improved high temperature mechanical properties, including improved high temperature creep resistance, relative to other ferritic stainless steels. The steel also should exhibit a good thermal expansion match with YSZ, the stabilized zirconia commonly used as electrolyte in SOFC. Thus, the steel is suitable for use in SOFC""s as current carrying interconnects and flow separators and may be used in place of ceramics. The steel also may be suitable for use in high stress, high temperature applications including, for example, oxygen sensor devices, certain chemical process, petrochemical, electrical power generation, and pollution control equipment, high temperature furnace hardware, and molten metal handling equipment.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend additional details and advantages of the present invention upon making and/or using the stainless steel of the present invention.